Power boilers have long been used by industries and utilities to produce steam for power production and for process requirements. These boilers come in many types and sizes ranging from 15 tons per hour steam production to 800 tons per hour or more. These boilers burn a variety of fuels including: Bark, sawdust, wood chips, and wood trimmings; wood or other biomass pellets; urban waste, refuse, and tire derived fuel (TDF); crushed coal, pet coke, fuel oils, and natural gas; sludge and fiber line rejects; sugar cane bagasse, pith, leaves, tops and other agriculture straw-based fuels; or other liquid, solid, or gaseous fuel, or a combination of fuels, and the solid fuels may have moisture content as high as 65%.
These boilers are typically constructed of heavy wall steel tubes welded side by side into straight wall panels that form the front, rear and side walls of the boiler. The lower portion of this box forms the combustion chamber of the boiler and is sometimes called the furnace. The tubes are typically 2″ to 3″ in diameter and spaced apart 2″ to 4″ center to center. If there are gaps between the tubes they are typically filled with steel strips about ¼″ thick by the width of the gap.
The entire panel is seal welded air tight. The lower ends of the wall tubes are welded into larger diameter horizontal header pipes that feed water to the walls. The tops of the wall tubes are also connected to larger diameter horizontal collector pipes that carry the water away from the walls to a steam drum, located at the top of the boiler. The front wall tubes are typically bent over to form the roof of the boiler and those tubes can terminate in a collector pipe or directly to the steam drum. Similarly the rear wall tubes are typically bent to create a “bullnose” or “nose arch” to direct combustion gasses across the convective section of the boiler and then terminate in a water drum, steam drum, or collector pipe at the top of the boiler. The top of the bullnose is usually at the elevation of the water drum. Downcomer pipes connect the steam drum or water drum at the top of the boiler to the header pipes at the bottom of the tube walls and feed water from the drum to the walls. The bottom of the boiler can be a travelling or vibrating grate, tilting grate, sloping grate, step grate, fluidized bed, or a Stepped Floor as described in U.S. Pat. No. 8,707,876. Fuel enters the boiler through a chute or chutes penetrating one or more walls of the boiler and may be broadcast into the boiler by a fuel distributor, for example, as described in U.S. Pat. No. 8,276,528. The fuel falls to the floor or grate where it is mixed with air and burns. The heat released by the burning fuel is absorbed by the wall tubes and heats the water in the walls, where the water expands thermally and starts to boil. The heated and boiling water is less dense than the water in the downcomer pipes therefore a natural circulation is created with hotter water rising in the tube walls and cooler water descending in the downcomer pipes. The natural circulation is an inherent safety feature of these boilers as the circulation rate increases as more fuel is burned and more heat released in the combustion chamber.
As the water circulates from the steam drum, down through the downcomers, up through the walls, and back to the steam drum, some or all of the steam is produced in the walls. Some of the steam may also be produced in the generating bank, sometimes called the boiler bank. In older two drum boilers, the generating bank is a set of tubes connecting the bottom of the steam drum to the top of a water drum, sometimes called a mud drum, located up to thirty feet or so directly below the steam drum. The steam and mud drums are typically cylindrical pressure vessels with their axes oriented horizontally and parallel to each other and to the front wall of the boiler. In a two drum boiler the steam drum is generally located directly above the water drum and there are hundreds of tubes connecting the two drums. The generating bank is arranged so that hot gasses from the furnace flow across the tubes and heat the water circulating inside. About half of the tubes in the generating bank of a two drum boiler are up flow tubes and the remainders are down flow tubes. The gas cools as it passes through the generating bank, therefore the first tubes the gas contacts (the front tubes as the gas flow through a boiler is generally front to back) are hotter and more boiling occurs in those tubes. The boiling water is less dense so the water circulates from the steam drum down through the rear tubes to the water drum and then up through the front tubes back to the steam drum. The steam drum is generally about half full of water with saturated steam being released at the surface. The steam goes through a set of moisture separators and then to the superheaters. In newer single drum boilers there is no water drum, instead, the generating bank is fed by external (non-heated) downcomers from the steam drum, and the water circulates down the downcomers and back up through all of the generating bank tubes to the steam drum. Single drum boilers are less expensive to build because the drums, especially with hundreds of tube penetrations, are the most expensive components. Steam and water drums are also expensive to build due to their large diameter, typically up to five feet or more, requiring commensurately thicker walls to withstand the internal pressure. Single drum boilers also have other advantages including more flexible arrangements for locating the steam drum and generating bank.
Some boilers also have sets of tubes located just at the furnace exit and arranged to cross the boiler at the top of the combustion chamber. These are called screen tubes or screens, and are often arrayed as platens in which several tubes are in close parallel arrangement, one on top of another, extending from the front or rear wall of the boiler through the opposite wall. These platens are generally separated 12″-15″ apart side to side and slope upward slightly to the other side of the boiler, or they may bend part way across the boiler and rise up vertically through the roof. The screen tubes are fed by external (non-heated) downcomers from the steam drum or water drum at their lower end and relieved back to the steam drum at their upper end. Water circulates from the steam drum or water drum through the screens and back up to the steam drum. The screens are located where the gasses are very hot and absorb heat by radiation and convection.
After the steam leaves the steam drum it goes to the superheaters. These are sets of tubes typically located at the top of the boiler, above the screen tubes and in front of the generating bank. The superheaters increase the temperature of the steam from the saturation temperature in the steam drum to the final temperature required by the process or the power plant.
The superheater tubes are typically arranged as vertical platens with up to a dozen tubes or more in close parallel arrangement front to back in each platen. There are many platens located across the width of the boiler with a spacing of 6″-15″ between platens.
There are frequently two or more superheater sections with connecting pipes and/or desuperheaters between the sections. Desuperheaters or attemporators control the final steam temperature by spraying water into the steam, or other means.
In top supported boilers the superheater tubes start at the top of the boiler and drop vertically to just above the bullnose then run up and down a number of times before exiting back through the roof. The steam passes through the superheaters just once therefore the superheaters are not part of the boiler circulation circuits.
After the boiler flue gasses exit the generating bank, they typically flow through an economizer or an air heater. Economizers are tube bundles either, in cross flow or parallel flow to the gas stream, through which the feedwater passes once and is heated and then goes to the steam drum. The feedwater flow is controlled to maintain the water level in the steam drum. Feedwater makes up for the steam that is produced and exits the boiler.
Upon entry into the drum, feedwater is baffled and mixes with some of the water already within the steam drum to flow to the downcomer pipes or downcomer tubes. This feedwater mixed zone has higher density, which provides the driving head for the natural circulation in the boiler. The economizer may be located immediately after the generating bank integral with the boiler, or it may be located downstream from a tubular air heater or a dust collector.
Some of these boilers are supported from underneath (ground supported) but most, especially larger boilers, are hung from the top and expand downward as they heat up. A top supported boiler requires a very strong and expensive external structure to support the boiler.
Boilers as described above have been in use for many years and the technology is very mature, but they are very expensive and have significant operational limitations. Mechanical grates suffer from poor reliability and grate fired boilers and fluidized bed boilers are limited in the temperatures they can tolerate in the lower furnace otherwise they will over heat the grate or sand bed. Mechanical grates also do a poor job of mixing the combustion air and fuel because of the high airflow dictated by the requirements to cool the grate or fluidize the sand bend. This leaves little setup flexibility to improve combustion mixing throughout the greater furnace volume above the lower furnace. Bubbling fluidized bed boilers and especially circulating fluidized bed boilers suffer from extensive erosion due to the sand particles flowing with the flue gases and can have problems with sand agglomeration, sintering, and glassification. These deficiencies are addressed with the introduction of stepped floor and fuel drying chute technologies as described in U.S. Pat. Nos. 8,707,876 and 8,590,463 respectively, and U.S. patent application Ser. No. 13/851,883 for a V-cell boiler.
A problem with existing boilers is that very light fuels, such as sugar cane pith and bagasse, are especially difficult to burn efficiently because of their light weight and high moisture content. Woody, straw and other solid fuels at some point reach a small size where they (cinders) become entrained in the combustion gas flow and fly out of the furnace before they completely burn. The unburned fuel and sand carryover can agglomerate and plug the convective sections (superheater, generating bank, and economizer), abrade the boiler tubes, increase particulate emissions, and reduce the thermal efficiency of the boiler.
Good control over carbon monoxide in biomass boilers is a major and historical problem because airborne cinders have insufficient residence time, and poor air/fuel volumetric mixing energies. Airborne fuel particles are still shielded fuel, where the outer skin surface must be first burned before the next layer of fuel becomes available.
Many boilers in the sugar cane and other agriculture crop processing industries suffer from low reliability, high maintenance costs, high particulate emissions, and poor thermal performance for these reasons. Sugar cane is the largest crop grown in the world. Historically the sugar cane waste (leaves and tops) has been burnt in the field, but new regulations are preventing that for environmental reasons.
While the prevention of in-field burning increases the availability of biomass for power production, as described previously, sugar cane biomass is also difficult to burn because it is light, very wet, and has high sand contamination. Sugar cane companies are realizing the profit potential to cogenerate electricity with their process demands, but available, affordable boilers use outdated designs that cannot keep up with modern market demands. Other agricultural crop processing industries such as rice, wheat straw, palm oil, etc., also face similar problems with their boilers. Furthermore, skilled labor costs are also rising around the world, and it is becoming more expensive to construct and erect conventional boilers.
Therefore a new boiler design is needed that will improve the combustion of sugar cane based fuels and other fuels, keep the acquisition and erection costs to a minimum, maximize boiler availability with low maintenance, reduce particulate emissions by first complete burning and second by multi-stage stripping/filtering, and provide the owners with a more profitable alternative over the life of the investment.